The invention relates generally to screws for use in a spinal implant system. The particular screw is a fixation screw that can be locked at a variable angle in a stabilizer anchor assembly. The screw includes restraining means that maintains the level of the anchor relative to the longitudinal axis of the screw.
Spinal implant systems have been developed for use in addressing certain spinal problems. These systems generally include an elongate stabilizer which is a rod, plate, or cable that is secured to a bone in a stabilizer anchor by means of a fixation member, in this case a screw. In some of these systems, the stabilizer anchor is a separate component from the screw in order to provide for a variable angle of the stabilizer fixation means relative to the angle of fixation. Often for such a system, the components are assembled prior to surgery. The present invention has application for such a system.
These implants have been developed over time to constitute very elegant constructs. The function dictates that the components are relatively easy to assemble and to manipulate and to surgically implant. The design requires minimization of size and invasion into the biological environment, while meeting the structural requirements to achieve the desired function of spinal alignment and stabilization depending on the medical indication. In addition, the construct has to be flexible in its use to account for differences in individuals, or for problems that may arise during the course of surgery. Spine surgery is by its nature intense and long surgery which requires great precision. It is critical that the implements that are developed for use by the surgeon are, given the design considerations, as efficient and easy to use as possible in order to facilitate the surgery.
In a preferred embodiment, the variable angle anchor of the present system can include a concave socket having a central opening through which the screw extends and which captures the convex head of the screw, and transverse to that opening, the anchor includes a channel for the stabilizer which resides above the screw head. Thus, the anchor and the head form a limited ball and socket joint to allow the anchor to have a variable angle on the screw. There can be means to lock the angle at a desired angle. Finally, the system can include means to secure the stabilizer in the anchor. In one particularly elegant design, the securing means can comprise compression means which locks the stabilizer in the anchor and which also locks the position of the anchor relative to the screw. The compression means illustrated with the assembly of the present invention is a nut which is screwed downwardly on external threads of the anchor. When screwed in place, the nut bears down on the stabilizer which in turn compresses the screw head in the anchor to form a locking interface and to lock the angular and radial position of the screw in the anchor. The bottom of the screw includes a machined, high friction surface which improves locking of the screw head in the socket. Of course, it should be understood that other locking or compression means could be used, for example, a locking disc, or a set screw, or other means which use internal threads or flanges on the anchor member.
The particular screw of the present invention is assembled into a cooperation with the stabilizer anchor to permit a variable angle of the anchor with respect to the screw which can subsequently be locked into position at a desired angle as previously discussed. However, the screw presents an advantage over the prior art insofar as the screw includes a restraining means which permits relatively easy assembly of the screw and anchor, but which inhibits the anchor from dropping on the screw to a position which may be difficult for the surgeon to access. The present invention also presents an advantage to variable angle spinal systems which maintain the freedom of movement between the anchor and the screw.
In a preferred embodiment, the restraining means is an annular collar having diamond point knurls which are sized to permit insertion of the screw and collar through the central opening of the anchor member, but where the collar will act to inhibit the anchor from slipping downward on the screw. The anchor actually floats on the screw and moves fairly freely, except the downward play is limited by the interaction of the annular shoulder of the central opening of the anchor and the restraining collar. Preferably at least a relevant portion of the screw head forms a ball and socket type interaction with the anchor opening. Thus, the screw head may include an generally outwardly rounded surface that forms an interface with an inwardly rounded surface of the anchor screw opening. In addition, there is preferably a mating interface between the screw and the anchor in order to ensure a locking cooperation between these two components. This mating interface may for example, be a high friction surface that may be formed as a result of the nature of the mating contoured surfaces, i.e., one surface may be slightly under specification, or alternatively, one or both of the mating surfaces may include a surface which has been worked to render it high friction, such as for example as is the case of the mating diamond point knurls that are shown. Other means of forming this mating locking interface are also contemplated, and include, for example, the use of a locking or pressure disc to effectively increase the frictional fit of the screw head in the socket of the anchor opening.
In the most preferred embodiment, the collar forms an annular barrier below a smooth inwardly rounding portion of the screw head which forms a neck between the screw head and the screw threads. The collar includes two rows of knurls which have a depth of the magnitude of about 0.01 inch and where the collar has a point to point diameter that is approximately the same magnitude larger than the opening in the anchor (or plate) through which it is inserted. This allows the knurls to asymmetrically deform during insertion so as to permit insertion but to inhibit the anchor from slipping below the collar. In the absence of the restraining means, it might be possible for the anchor to slip downward on the screw to a position where it is difficult for the surgeon to reach. This situation could arise for example, where a vertebrae is mis-aligned in the anterior direction, and the screw might not be screwed fully into position. In this instance, the bone surface doesn""t act to hold the anchor close to the head of the screw.
It is an object of the invention to provide a bone screw that can be used with a stabilizer including but not limited to a rod or plate or other construct component that is secured to the bone by the screw where the screw includes a restraining collar below the head of the screw in a necked area upward of the screw threads.
It is a further object of the invention to provide a spinal screw that is used in conjunction with a stabilizer anchor where the screw is inserted through an opening in the anchor along the longitudinal axis of the screw, where the screw includes means to inhibit the anchor from slipping longitudinally downward along the screw. In a preferred embodiment, the means to inhibit the downward motion comprises a collar having deformable projections that are collectively close to or slightly larger than the size of the opening in the anchor for the screw to be inserted through. For example, these projections can be diamond point knurls that allow for the screw to be inserted through the opening to the necked portion of the screw, but which maintain the anchor above the collar notwithstanding the fact that the knurls were deformed slightly during assembly with the anchor. Of course, other deformable configurations, such as spiral ridges or splines, or threads could also be used with the collar as the restraining means. Alternately, an asymmetrical opening or key could be used with a corresponding keyway on the collar (or anchor respectively.)